An electrical filter is a two-port circuit that has a desired specified response to a given input signal. Many filters are used to allow certain frequencies to be transmitted to an output load while rejecting the remaining frequencies. The use of low pass, high pass and bandpass filters in microwave systems is well-known to separate frequency components of a complex wave. For instance, microwave filters are commonly used in transmit paths to suppress spurious radiation or in the receive paths to suppress spurious interference.
The design of microwave filter circuitry is complicated by the fact that conventional electronic components do not retain their basic electric properties when operated at microwave frequencies. Thus, specialized electric circuit techniques which exploit both the electric and magnetic properties of the wave are commonly employed. For example, the conductors which carry microwave signals between components often take the form of waveguides. Waveguides are guided field structures commonly having either rectangular or circular cross sections, usually constructed of a highly conductive material and to a high degree of precision. The effects of capacitance and inductance are introduced into guided field structures through which the microwave signals pass by sitting posts, stubs, annuli and so on. The physical dimensions of these devices and their position in relation to the guided field structure determine the type of effect they are to produce. One such effect would be the passage of only a desired microwave signal band through the waveguide to realize a bandpass filter.
Waveguide filters may operate in a single mode or may be of a multi-mode type. With the multi-mode filters of previous designs, the existing modes are synchronously tuned to augment the performance of filters with a single passband. Two of the earliest descriptions of a two mode filter is set forth in an article by Ragan, entitled "Microwave Transmission Circuits", Volume 9 of the Radiation Laboratory Series, McGraw Hill, 1948, pp 673-679, and an article by Wei-guan Lin, entitled "Microwave Filters Employing a Single Cavity Excited in more than One Mode", Journal of Applied Physics, Vol. 22, No. 8, August 1951, pp. 989-1001, wherein a five mode single cavity filter is described.
Many other articles about multi-mode filters, with a single passband, have appeared in the literature, including: "Nonminimum-Phase Optimum-Amplitude Bandpass Waveguide Filters", A. E. Atia and A. E. Williams, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-22, No. 4, April 1974, pp. 425-431; "Mixed Mode Filters", D. A. Taggart and R. D. Wanselow, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-22, No. 10, October 1974, pp. 898-902; "Dual Mode Canonical Waveguide Filters", A. E. Williams and A. E. Atia, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-25, No. 12, December 1977, pp. 1021-1026; and "Filter Design Using In-Line Triple-Mode Cavities and Novel Iris Couplings", U. Rosenberg and D. Wolk, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-37, No. 12, December 1989, pp. 2011-2019.
All of the filters described above have the common characteristic of having a single passband. Such filters are useful to filter the output of a transmitter which outputs a single frequency, however, when these filters are employed with transmitters that generate more than one frequency, the design becomes more complicated.
Referring to FIG. 1, there is shown a conventional prior art two frequency system 10 that employs two transmitters 12, 14 and a three port diplexer 20 to combine their outputs. The first transmitter 12 is coupled to the first filter 16 via microwave path D and the second transmitter 14 is coupled to the second filter 18 via microwave path C. The microwave paths will most likely be in the form of waveguides, which as discussed, are well-known in the art. The first filter 16 is coupled to one input of the diplexer 20 via microwave path A and the second filter 18 is coupled to the other input of the diplexer 20 via microwave path B. The lengths of the microwave paths C, D which couple the transmitters 12, 14 to their respective filters 16, 18 are not considered critical with regard to the operating frequencies of the transmitters 12, 14. On the other hand, the lengths of the microwave paths A, B, which emanate from the filters 16, 18 to the inputs of the diplexer 20 are critical. That is, exact phase lengths of the paths A, B must be established and maintained for proper operation of the system 10. If the operating frequencies of either, or both transmitters 12, 14 are changed, then either the length of path A, path B or both paths A and B must be changed.
When two frequencies are generated by a common source, the design of an output filter system using conventional techniques is more complex than a single frequency system. Referring to FIG. 2, there is shown prior art of an output filter system 22 which receives two frequencies of microwave signals generated from a common source (not shown). As can be seen, the filter system 22 employs two three port junctions 24, 25 for transporting the RF energy to and from the first filter 26 and the second filter 28. The filter system 22 of FIG. 2 contains four critical length microwave paths E, F, G, H. Paths E and F connect the first filter 26 with the first and second three port junctions 24, 25, respectively. Paths G and H connect the input and output of the second filter 28 to the respective three port junctions 24, 25. Exact phase lengths of each path E, F, G, H must be established and maintained for proper operation of the filter system 22. Thus, if either frequency in the system 22 needs to be changed, then two of the four path lengths must be modified. If both frequencies are changed, then, all of the path lengths E, F, G, H will also require modification.
It is therefore an object of the present invention dual passband microwave filter to provide a single structure microwave filter without the critical path lengths that require modification when frequencies are altered.
It is further objective of the present invention dual bandpass microwave filter to provide a dual bandpass filter that has a simpler structure, reduced size and lower cost structure than comparable prior art filters.